Monte Carlo simulations of the crystallization of isotactic polypropylene

Abstract

We model the temperature dependent conformational changes and orientational and translational ordering of isotactic polypropylene melt in the vicinity of its melting temperature. We use the coarse grained second nearest neighbor lattice model, which has previously been shown to properly model the liquid state thermodynamics of this material. Equilibrium Monte Carlo (MC) simulations show the existence of two characteristic temperatures. Above T = 450 K the chains assume standard Gaussian statistics, but below this temperature we find that helix formation is thermodynamically favored even though the melt is still disordered. Both the helix length and fraction of chain monomers involved in helices increase monotonically with decreasing temperature, and below T = 385 K this material undergoes crystallization. Our results are consistent with recent simulations, which suggest that isothermal crystallization is temporally preceded by the formation of a smectic phase: we argue that the helices are long enough under these conditions to support liquid crystallinity. The crystallinity then evolves in these liquid crystalline "droplets". Further, we can understand the recent results of de Jeu et al. who had found that isotactic polypropylene chains formed a smectic-like phase under the action of a planar shear even at temperatures above the equilibrium melting point. In summary, these findings support the following physical picture: the unusual spinodal decomposition phenomena seen in the early stages of isothermal crystallization, which have been assigned to the formation of small liquid crystal droplets, have equilibrium signatures above the melting point where locally rigid chain sequences become thermodynamically stable. © 2006 Wiley Periodicals, Inc.